The access network is the last drop between a service provider and a subscriber. This drop, for connecting the subscriber and the service provider, is still today mainly copper dominated in most regions of the world. However, in the future demand for Fiber-To-The-Home, FTTH, is expected to increase based on increased bandwidth requirements. FTTH is feasible as point-to-point, P2P, or point-to-multipoint, P2MP, architectures. Physical P2MP topologies may be achieved using Passive Optical Networks, PONS. PONS may also be used for Fiber-To-The-x, FTTx, applications.
While PONS have many promising advantages, such as large reduction of the number of fibers from the network core, low power consumption, sharing of active equipment on the network side, reliability, upgradability and possible cohabitation of several operators on the same passive infrastructure, some problems and drawbacks, compared to other technologies, are still unsolved. One particular challenge for the network operator is to maintain and monitor the PON and to locate failures of equipment or fiber.
With reference to FIG. 1, a block diagram of one simple example of a PON, will now be described. The PON comprises a central office node 100. The Central office node 100 comprises an Optical Line Terminal, OLT, 101 and OTDR functionality for issuing Optical Time Domain Reflectrometry, OTDR, measurements. The OLT 101 normally also comprises monitoring functionality for maintaining the PON. One or more passive Remote Nodes, RN, 110 are connected to the OLT 101. The RNs are normally 110 having a filter and a splitter (not shown). The OLT 101 is connected to the splitter via the filter for routing optical signals from the RN 101 to the Optical Network Terminals, ONTs, also called Optical Network Units, ONUs, 120a-n. 
Optical Layer Supervision, OLS, contains a set of capabilities relating to the measurement and reporting of the state of the optical link for Passive Optical Networks, PONS, such as Gigabit capable PONS, G-PONS, or 10G-PONS also known as XG-PONs. One necessary part of OLS is transceiver parameter monitoring which also is known as Optical Transceiver Monitoring, OTM. OTM may for example report the temperature, voltage, source bias current and/or received/transmitted power of a transceiver in the PON.
OTM measurements, such as received power of the transceiver, combined with OTDR, may enable the Central Office to detect if there is a faulty fiber link in the PON. Moreover, such combination may also enable increased capabilities to localize the fault.
One solution for measuring the received optical power at the ONT is to branch and measure a small amount of the received optical power. The branch and measurement is normally done within the ONU. With reference to FIG. 2, a block diagram illustrating an example configuration of an ONT comprising a branch solution for measuring the received optical power, according to the prior art will now be described. The ONT 200 is connected to the Optical Distribution Network, ODN, via a fibre 201. The fibre 201 is branched 204 in the ONU 200. In this example, a first portion of the received optical power is connected to a Reflective Semiconductor Optical Amplifier, RSOA, 202. The RSOA 202 is a gain medium which is reflective. The RSOA also erases the downstream signal and re-modulates the optical signal to carry the upstream data signal. In this description, the term ODN shall mean all equipment and optical fibre between the OLS and the ONT. I.e. the optical light signal is travelling from the OLT to the ONT through the ODN and back. Examples of equipment in the ODN are splitters, filters and switches.
In FIG. 2, a second portion is branched to a receiver 203, typically a semiconductor circuit capable of measuring the received optical power, e.g. a photodiode. Preferably, 10% of the downstream optical signal is branched, i.e. becomes the second portion, and measured in order to get a reliable result indicating the state of the drop link. Even if a very small amount is branched, such as 1%, the power loss over time will become significant. Consequently, this solution is associated with undesired Operational Expenses, OpEx, for the PON owner.
Hence, there is still a demand for simple, reliable, effective and low cost solutions for measuring and reporting the received optical power at the ONUs in a PON.